Were you ever not able to use a tool because it lost all its durability? Even in Erinn, things get old and rusty.
Each item has a different durability, some higher and some lower than others.
It determines how long the item will last.
In the item description, the number on the left indicates the current durability and the one of the right refers to the item's maximum durability.
The durability of an item will decrease the more you use it.
Once the durability reaches 0, it will tell you that the item can no longer be used.
You must go find the NPC that can repair the item.
Go to the Blacksmith's or the Weapons Shop for weapons or armor, and the Inn or the Clothing Shop for clothes.
For any other trinkets, you can repair it at the General Shop.
Each NPC's Repair skill level is different so their success rate will vary.
For example, Nora, who works at the Inn, can repair clothes with a 94% success rate.
When you repair an item, you have an option of choosing a 1 Point Repair or a Complete Repair.
If the repair fails, the max durability of the item will decrease. But please don't blame the repairer, they're human too!
If you apply the Blessing Potion, which you can get from the Church (you can only get it by doing church work), on your item, it will not only maintain the life of the item longer but it will also increase the success rate of your repair as well.
So, in the end, you can reuse items that have been worn out.
However, if an item is precious to you, rather than using it all the time and repairing it, it is probably best to apply the Blessing Potion on it and use it occasionally. That way, the item will last longer.